Forward/reverse transmission for scale model vehicle

ABSTRACT

A forward/reverse transmission, shiftable under servo-control, includes a movable clutch bell and a free-wheeling, one-way centrifugal lock-out clutch that permits a gear change from forward to reverse or reverse to forward only while the engine is operating at or below idle speed. The centrifugal lock-out clutch includes a spring-loaded pawl which is yieldably biased by a compression spring and held in a non-interfering, shiftable position when the engine is being operated at or below idle speed. As the engine rpm increases above idle, the centripetal acceleration force overcomes the bias force of the compression spring and the pawl is extended radially outwardly for engagement with a torque transfer pin. In the radially extended position, the pawl is disposed for interference contact against the clutch bell housing, thus preventing clutch engagement/disengagement. In an alternative embodiment, the centrifugal lock-out clutch includes a pair of spring-loaded friction shoes which are biased in the retracted (non-interfering) position by torsion springs.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to scale model racing vehicles,and in particular to radio-controlled model racing cars that are poweredby miniature glow plug internal combustion engines.

[0002] Radio-controlled model racing is a popular hobby sanctioned byRadio-Operated Auto Racing, Inc., among other rule making organizations.Competition events feature model cars, model aircraft and model boats.Racing heats are generally staged on a closed-circuit race course andrequire each competing model to complete as many laps as possible withina specified time period, with the model completing the largest number oflaps being declared the winner. Some racing events are conducted over anunimproved off-road outdoor area where the model car must be steeredcarefully to avoid collision with obstacles. When a collision occurs, itmay be necessary to drive the model car in reverse to clear the obstaclebefore the race can be continued.

[0003] Each model is controlled in terms of steering, throttle andforward/reverse travel by low-power, digitally encoded radio-frequencycommand signals at a dedicated frequency generated by a hand-held remotecontrol transmitter. Each model is equipped with an on-board servo andradio receiver that is tuned to the same frequency as the transmitter tocause the model to turn, increase speed, slow down and reverse directionas commanded by the operator.

[0004] There are two main categories of radio-controlled scale modelvehicles, battery-powered and fuel-powered. The prime mover in abattery-powered vehicle is an electric motor, while the prime mover in afuel-powered vehicle is an internal combustion engine. Sincefuel-powered vehicles generally do not have an on-board electricalgenerating system, a small battery is included to provide electricalpower for operating on-board radio system components. The on-board radiosystem components include a receiver and servo motors. Conventionalbattery-powered vehicles achieve reversal of the prime mover (electricmotor) by reversing the polarity of the applied voltage. Conventionalfuel-powered vehicles have no method for reversing the internalcombustion engine, and thus are not operable in reverse.

[0005] One conventional radio-controlled scale model vehicle is equippedwith an on-board battery and a DC electric motor for cranking theinternal combustion engine during starting, and also for providingmotive power during reverse travel operation. The internal combustionengine, which is not reversible, provides operating power for the modelvehicle only during forward travel operation. The forward gear isdisengaged and the engine is brought to idle under servo-control topermit transfer to the DC electric motor through a power transferlinkage and a reverse gear so that the model vehicle can be propelled byelectrical power in the reverse direction.

[0006] It will be appreciated that the sequential shifting operation,which requires transition to idle speed, disengagement of the engine andengagement of the electric drive motor, imposes an undesirable timedelay before the vehicle motion can be completely reversed.Additionally, if direction of travel is reversed while being operated ata high rate of speed, the gearing and/or power transfer linkage can bedamaged.

[0007] Accordingly, there is a need for a simple, rapid and reliablemeans for selectively reversing the forward driving torque produced by aprime mover, for example an internal combustion engine or inertialflywheel motor that cannot be reversed, into reverse driving torque,thus eliminating the need for an on-board battery and electric drivemotor for reverse travel. Additionally, a shiftable transmission isneeded for use in combination with a radio-controlled scale modelvehicle in which shifting from forward to reverse is performed withoutdamaging the transmission gear train or linkage.

BRIEF SUMMARY OF THE INVENTION

[0008] According to the present invention, a radio-controlled modelvehicle includes a shiftable transmission that is powered by a miniatureinternal combustion engine during both forward and reverse travel. Thetransmission includes a forward/reverse torque transfer assembly that isshiftable under the control of a servo-driven shuttle. Theforward/reverse torque transfer assembly includes a shiftable clutchbell coupled to the shuttle and a centrifugal lock-out clutch thatpermits a gear change from forward to reverse or reverse to forward onlywhile the prime mover (the internal combustion engine) is operating ator below idle speed. Drive train shock loading and damage to thetransmission and its associated parts are avoided by preventing gearchanges for any engine speed above idle.

[0009] According to one aspect of the invention, the centrifugallock-out clutch includes a spring-loaded pawl which prevents gear changewhile the engine is being operated above a predetermined idle speed. Forengine operation or at or below idle speed, the pawl is retracted by abias compression spring to a non-engaging position. As the engine rpmincreases above idle, the inertial force developed by centripetalacceleration overcomes the bias force of the compression spring and thepawl is extended radially outwardly for positive engagement against atorque transfer pin carried on the clutch bell, and driving torque istransmitted to the wheels.

[0010] Shifting movement of the shuttle and the clutch bell areprevented by interference contact of the inertially extended pawlagainst the clutch bell housing when the engine is operating at speedsabove idle. Shuttle shifting and clutch engagement/disengagement areautomatically enabled when the engine speed drops below idle, as thebias spring moves the pawl from the shift-blocking position into theretracted, non-interfering position, so that the clutch bell can beshifted into or away from the pawl engaging position.

[0011] According to another aspect of the invention, the centrifugallock-out clutch includes a pair of spring-loaded friction shoes. At idlespeed, the friction shoes are held in the retracted (non-interfering)position by torsion bias springs, and the clutch bell is free to eitherforward or reverse shift to an engagable torque transfer position overthe friction shoes. As the engine rpm increases above idle, the inertialforces developed by centripetal acceleration overcome the yieldablerestraining force of each torsion spring, thus extending the frictionshoes radially outwardly into frictional engagement against the clutchhousing and transferring driving torque to the wheels. The clutch bellcannot be disengaged and shifted from one position to the other as longas the engine rpm remains above idle.

[0012] In each embodiment, the position of the clutch bell in relationto the shoe/pawl inertial stop apparatus determines whether the clutchis permitted to engage/disengage the forward gear or the reverse gear.If the clutch bell is not positioned over a shoe or pawl and the engineis operating above idle rpm, the radial extension of the shoe/pawlblocks axial shifting movement of the shuttle and clutch bell.Consequently, for any engine speed above idle, a gear change fromforward to reverse or reverse to forward is not allowed.

BRIEF DESCRIPTION OF THE DRAWING

[0013] The accompanying drawing is incorporated into and forms a part ofthe specification to illustrate the preferred embodiments of the presentinvention. Various advantages and features of the invention will beunderstood from the following detailed description taken in connectionwith the appended claims and with reference to the attached drawingfigures in which:

[0014]FIG. 1 is a perspective view of a radio-controlled scale modelracing car that is powered by a miniature internal combustion engine;

[0015]FIG. 2 is a simplified electro-mechanical block diagram of aservo-mechanism and gear train assembly;

[0016]FIG. 3 is a perspective view of a forward/reverse transmissionconstructed according to a first preferred embodiment of the presentinvention;

[0017]FIG. 4 is a top plan view thereof, with the clutch bell engaged inthe forward drive position;

[0018]FIG. 5 is a view similar to FIG. 4, showing the clutch bellengaged in the reverse drive position;

[0019]FIG. 6 is a front elevational view, partly in section, of apositive engagement clutch which incorporates a spring-loaded pawl;

[0020]FIG. 7 is a right side elevational view of the positive engagementclutch and spring-loaded pawl embodiment taken along the line 7-7 ofFIG. 6;

[0021]FIG. 8 is a right side elevational view of the clutch bell shownin FIG. 5;

[0022]FIG. 9 is a sectional view of the clutch bell shown in FIG. 8;

[0023]FIG. 10 is a perspective view of a power transmission including afriction clutch which utilizes spring-loaded friction shoes;

[0024]FIG. 11 is a top plan view thereof, with the clutch bell shiftedand frictionally engaged in the reverse travel direction;

[0025]FIG. 12 is a view similar to FIG. 11 in which the clutch bell isshifted and frictionally engaged in the forward direction of travel;

[0026]FIG. 13 is a side elevation view of an alternative embodiment ofthe centrifugal lock-out clutch, showing the spring-loaded frictionshoes in elevation;

[0027]FIG. 14 is a simplified right side elevational view thereof, shownpartly in section;

[0028]FIG. 15 is a perspective view of a power transmission whichincludes a pair of free-wheeling one-way clutch assemblies;

[0029]FIG. 16 is a sectional view, partially broken away, of thefree-wheeling one-way clutch assembly; and,

[0030]FIG. 17 is an exploded perspective view of the power transmissionshown in FIG. 16.

DETAILED DESCRIPTION OF THE INVENTION

[0031] Preferred embodiments of the invention will now be described withreference to various examples of how the invention can best be made andused. Like reference numerals are used throughout the description andseveral views of the drawing to indicate like or corresponding parts.

[0032] Referring now to FIG. 1, a scale model racing car 10 includesfront steering wheels 12, 14 and rear driving wheels 16, 18 mounted on achassis 20. The scale model racing car 10 is powered by a miniaturefuel-burning internal combustion engine 22 which is a compression(glow-plug) or spark ignition engine having a displacement of 2.5 cc to23.0 cc and providing a power output in the range of 0.5 h.p. to 5.0h.p.

[0033] Referring now to FIG. 2 and FIG. 3, the rear drive wheels 16, 18are mechanically attached to output shaft stub sections 24A, 24B whichare coupled to the output gears of a differential gear unit 25. Thedifferential gear unit 25 is driven by a main drive shaft 24 which iscapable of rotation in clockwise and counterclockwise directions, forpropelling the rear wheels in the forward and reverse directions.

[0034] The main drive shaft 24 extends along the length of the model carand is connected at the front of the model car to a differential gearunit 27. The front wheels 12, 14 are connected to the output gears ofthe differential gear unit 27 by output shaft stub sections 24C, 24D,respectively.

[0035] The engine 22 is capable of powering the scale model racing carat speeds in the range of 20 mph to 80 mph. The output torque developedby the engine 22 is delivered through a power output shaft 26. As shownin FIG. 2, an onboard DC electric motor 28 is coupled by a pinion gear30, an idler gear 32 and a spur gear 34 to the power output shaft 26 forstarting the engine 22. According to this arrangement, the engine 22includes a conductive glow plug connector 36 which, along with the DCelectric motor 28, is connectable to an external DC electric powersupply for cranking and starting the engine 22.

[0036] After starting, the external power supply is disconnected and themodel car 10 is thereafter powered in the forward and reverse directionssolely by the internal combustion engine 22. It will be appreciated thatprime mover operation of the internal combustion engine 22 is notreversible, and the model car 10 does not carry an onboard battery packfor supplying operating power to the DC electric motor. Consequently,the model car is powered solely by the internal combustion engine 22after starting for reverse as well as forward operation.

[0037] The rotary power output of the internal combustion engine 22 isapplied to the rotatable main drive shaft 24 through a transmissionassembly 38. Referring now to FIG. 2 and FIG. 3, the rotary outputtorque developed by the internal combustion engine 22 is applied to thetransmission assembly 38 through an input pinion gear 40 which drives aprimary input spur gear 42. The spur gear 42 is attached to a primarytorque transfer shaft 44 which is mounted for rotation about itslongitudinal axis A.

[0038] The rotary torque transmitted through the spur gear 42 to thetorque transfer shaft 44 is selectively applied to the main drive shaft24 through a forward drive spur gear 46 or a reverse drive spur gear 48upon engagement with a clutch bell 50. The forward drive spur gear 46and the reverse drive spur gear 48 are rigidly attached to opposite endsof the torque transfer shaft 44. Thus the rotary power output ofinternal combustion engine 22 can be selectively applied through eitherthe forward drive spur gear 46 or the reverse drive spur gear 48,according to the shifted position of the clutch bell 50, as describedbelow.

[0039] Referring again to FIG. 2, the rotary output torque produced bythe internal combustion engine 22 is selectively converted to forwarddrive torque by a one-way pawl clutch 52 and a forward drive spur gear54 which is attached to the clutch 52. The forward drive spur gear 54and one-way forward clutch 52 are free-wheeling with respect to the maindrive shaft 24, except when the clutch bell 50 is engaged with theclutch 52.

[0040] Referring to FIG. 8 and FIG. 9, the clutch bell 50 includes shaftinsert collar portions 56, 58 which are intersected by an axial bore 59that has flat side faces 57 which are engagable with a conformed torquetransfer shaft portion 24T of the main drive shaft. During shiftingmovement of the clutch bell 50, the insert collars 56, 58 ride on thetorque transfer shaft portion 24T until the clutch bell 50 is carriedinto a position overlying the forward clutch 52 (FIG. 4) or the reverseclutch 60 (FIG. 5).

[0041] For forward travel operation, the clockwise output torque of theengine 22 is applied through the primary torque transfer shaft 44 to themain drive shaft 24 by an idler spur gear 64 which is meshed with theforward drive spur gear 46 and the forward output spur gear 54.According to this arrangement, counter-clockwise rotation of the torquetransfer shaft 44 is converted into clockwise rotation of the main driveshaft 24, thereby producing forward (F) rotation of the front and rearwheels.

[0042] The rotary output torque produced by the internal combustionengine 22 is selectively converted to reverse drive torque by a one-wayreverse clutch 60 and a reverse drive spur gear 62 which is attached tothe reverse clutch 60. For reverse travel operation, the clockwiseoutput torque of the engine 22 is applied through the torque transfershaft 44 to the drive axle 24 by direct engagement of the reverse drivepinion 48 with the reverse drive spur gear 62. The one-way reverseclutch 60 and output spur gear 62 are free-wheeling with respect to themain drive shaft 24, except when the clutch bell 50 is engaged with thereverse clutch 60.

[0043] Referring now to FIG. 2, FIG. 4, FIG. 5 and FIG. 6, the clutchbell 50 is slidable along the main drive shaft 24 to the forward driveposition (FIG. 4) in which the clutch bell is engaged with the forwardclutch 52 for transmitting torque to the rotary axle in the forwarddirection, and is slidable to the reverse drive position (FIG. 5) inwhich the clutch bell is engaged with the reverse clutch 60 fortransmitting torque to the torque transfer shaft 44 in the reversedirection. The clutch bell 50 is shiftable along the main drive shaft 24by a servo-actuated shuttle 66. The shuttle is mounted for slidingmovement along a guide rail 68, and is attached to the clutch bell 50 bya shift arm 70.

[0044] The shuttle 66 is driven by a battery-operated servo 72 whichincludes a rotary actuator arm 74. The shuttle 66 and the rotaryactuator arm 74 are coupled together by a linking arm 76. The servoactuator arm shifts the shuttle 66 and clutch bell 50 between theforward and reverse positions in response to digitally encodedradio-frequency command signals received by a high frequency, multiplechannel receiver 78. The receiver 78 decodes the radio command signalsand outputs control signals (FORWARD and REVERSE) to the servo 72through a multiple conductor signal cable 80.

[0045] According to an important feature of the invention, the forwardclutch 52 and the reverse clutch 60 are equipped with spring-loadedpawls 82, 84, respectively (FIG. 6 and FIG. 7), that permit a gearchange from forward to reverse or reverse to forward only while theinternal combustion engine 22 is operating at or below idle speed. Whenthe spring-loaded clutch pawls 82, 84 rotate outwardly in response toincreased engine rpm, they mechanically engage torque transfer pins 61,63 (FIG. 3 and FIG. 8), thereby mechanically connecting the forwarddrive spur gear 54, the forward clutch 52 and the clutch bell 50 intoforward driving engagement with the main drive shaft 24. According tothis arrangement, drive train shock loading and damage to thetransmission and its associated gears are avoided by preventing gearchanges for any engine speed above idle.

[0046] Referring now to FIG. 2, FIG. 6 and FIG. 9, for engine operationat or below idle speed, the pawl 84 is retracted by a bias compressionspring 86 to a non-interfering position. As the engine rpm increasesabove idle, the force developed by centripetal acceleration overcomesthe bias force of the compression spring and the pawl 84 rotates about apivot pin 88 and is extended radially outwardly (to the dashed-lineposition) into driving engagement with one of the torque transfer pins61, 63 and partially overlaps (in radial extent) the housing face 50F ofthe clutch bell 50. That is, shifting extension movement of the shuttle66 into or out of engagement with the clutch 52 is prevented byinterference contact of the inertially extended pawl 84 (in the dashedline blocking position) against the clutch housing 50F when the engineis operating at speeds above idle.

[0047] When the engine speed drops below idle, the centripetal forcediminishes and is overcome by the bias spring 86 which moves the pawl 84from the blocking/torque transfer position (as indicated by the dashedline) into the retracted, non-interfering shifting position as shown inFIG. 6. The bias restoring force of the compression spring 86 is appliedthrough a ball-bearing 90 that is slidably received for reciprocalmovement through a cylindrical bore 92 drilled through the clutch body60. The ball-bearing 90 is captured between the compression bias spring86 and the pawl 84. The magnitude of the restoring bias force developedby the compression spring 86 is adjusted by a set screw 94 to a levelpermitting the bias spring 86 to drive the pawl 84 to the fullyretracted position (at idle speed) within a clutch slot 96, as indicatedby the solid line position in FIG. 6 and FIG. 7.

[0048] According to an alternative embodiment as shown in FIGS. 10-14,each centrifugal lock-out clutch includes a pair of spring-loadedfriction shoes 102, 104 and 106, 108, respectively, for frictionaltorque transfer engagement against the clutch bell 50. Torque transferpins are not used in this embodiment. In this arrangement, each frictionshoe is biased to the retracted position (as indicated by the solidlines in FIG. 13 and FIG. 14) by torsion springs 110 and 112,respectively. The torsion springs are fitted about retainer pins 114,116, respectively. The radially inner end portions 110A, 112A of thetorsion springs are received within retainer pockets 118, 120,respectively. The opposite end portions 110B, 112B are received withinretainer slots formed across the outer periphery of each friction shoe.

[0049] Referring to FIG. 10 and FIG. 13, for engine operation at orbelow idle speed, the friction shoes 102, 104 are retracted by thetorsion springs 110, 112 to a non-interfering position. As the enginerpm increases above idle, the force developed by centripetalacceleration overcomes the bias force of the torsion springs 110, 112and the friction shoes rotate about the pins 114, 116 and are extendedradially outwardly (to the dashed line position) as the bias force ofeach torsion spring is overcome. As the friction shoes extend radiallyoutwardly, each shoe engages the clutch bell 50 in frictional torquetransfer contact, and also partially overlaps the housing face 50F ofthe clutch bell 50. Shifting movement of the clutch bell 50 is preventedby interference contact of the radially extended friction shoes 102, 104(in the dashed line blocking position) against the bell housing 50F(FIG. 9) when the engine is operating at speeds above idle.

[0050] When the engine speed drops below idle, the centripetal forcediminishes and is overcome by the bias force exerted by the torsionsprings 110, 112 which moves each friction shoe from the radiallyextended position (as indicated by the dashed line) to the retracted,non-interfering position (as indicated by the solid line), as shown inFIG. 13.

[0051] Referring now to FIG. 15, FIG. 16 and FIG. 17, the rotary torqueoutput of the internal combustion engine 22 is applied to the main driveshaft 24 through a transmission assembly 120 which utilizes one-way,direct clutch assemblies 122, 124. The transmission assembly 120 isconstructed substantially identically to the transmission assembly 38,except that intertial clutch elements, torque transfer pins and biassprings are not utilized. In this alternative embodiment, each one-wayclutch includes clutch slips 126 (FIG. 16) that are mounted for pivotalmovement on a roller bearing race 128. The one-way slip clutches 122,124 are mounted within collars 130, 132 that are rigidly attached to theforward drive spur gear 54 and to the reverse drive spur gear 62,respectively. Each clutch collar 130, 132 surrounds a cylindrical pocket134 in which each clutch assembly 122, 124 is received.

[0052] Referring to FIG. 16 and FIG. 17, when the clutch bell 50 isshifted to the reverse drive position, the clutch bell insert collar 58is inserted into the clutch pocket of the one-way clutch 124. Thepivotal slips 126 provide one-way wedging, torque transfer engagementbetween the clutch bell insert collar 58 and the spur coupling collar132 in response to counter-clockwise rotation of the main drive shaft24. The pivotal slips 126 disengage to permit free wheeling rotation ofthe clutch bell insert collar 58 on the roller bearings 136 in responseto clockwise rotation of the main drive shaft.

[0053] Upon reverse gear engagement, as shown in FIG. 15, the shaftinsert collar 58 carried by the clutch bell 50 engages the pivotalclutch slips 126 (FIG. 16), thereby releasably connecting the reversespur gear 62, the reverse clutch 124 and the clutch bell 50 into one-wayreverse drive, torque transfer engagement with the main drive shaft 24.The output torque of the engine 22 is reversed by the drive pinion 48which is meshed with the reverse spur gear 62. According to thisarrangement, counterclockwise rotation of the torque shaft 44 isconverted into clockwise rotation of the drive shaft 24 and drivewheels, thereby producing reverse (R) rotation of the drive wheels.

[0054] Although the invention has been described with reference tocertain exemplary arrangements, it is to be understood that the forms ofthe invention shown and described are to be treated as preferredembodiments. Various changes, substitutions and modifications can berealized without departing from the spirit and scope of the invention asdefined by the appended claims.

I claim:
 1. In a scale model vehicle including a chassis, a main driveshaft mounted for clockwise and counterclockwise rotation on thechassis, drive wheels coupled to the main drive shaft for propelling thevehicle in forward and reverse directions, an internal combustion enginehaving a rotary power output shaft, and a transmission assembly coupledto the rotary power output shaft for transmitting torque from the engineto the main drive shaft, the transmission assembly comprising: a firstclutch coupled to the main drive shaft for transmitting torque in aclockwise direction; a first spur gear attached to the first clutch; asecond clutch coupled to the main drive shaft for transmitting torque ina counterclockwise direction; a second spur gear attached to the secondclutch; and, a clutch bell coupled in torque transmitting engagementwith the main drive shaft, the clutch bell being movable along the maindrive shaft to a forward drive position in which the clutch bell isengagable with the first clutch for transmitting torque to the maindrive shaft in the clockwise direction, and movable to a reverse driveposition in which the clutch bell is engagable with the second clutchfor transmitting torque to the main drive shaft in the counterclockwisedirection.
 2. The combination as set forth in claim 1, including: atorque transfer shaft mounted on the chassis for rotation about alongitudinal axis; a first pinion gear attached to the torque transfershaft and coupled in torque transfer engagement with the first spurgear; a second pinion gear attached to the torque transfer shaft; and,an idler gear coupled in driving engagement with the second pinion gearand second spur gear.
 3. The combination as set forth in claim 1,including: a first pawl mounted on the first clutch for pivotal movementfrom a radially retracted position to a radially extended position; asecond pawl mounted on the second clutch for pivotal movement from aradially retracted position to a radially extended position; and, firstand second bias springs coupled to the first and second pawls,respectively, for urging the pawls toward the retracted positions andyieldably opposing radial extension of the first pawl and second pawl,respectively.
 4. The combination as set forth in claim 3, including: atleast one torque transfer pin attached to the clutch bell in a positionenabling torque transfer engagement with the first pawl or the secondpawl when the clutch bell is disposed in either the forward driveposition or in the reverse drive position.
 5. The combination as setforth in claim 3, wherein each bias spring comprises a torsion spring.6. The improvement as set forth in claim 1, including: a first frictionshoe mounted on the first clutch for pivotal movement from a radiallyretracted position to a radially extended position; a second frictionshoe mounted on the second clutch for pivotal movement from a radiallyretracted position to a radially extended position; and, first andsecond bias springs coupled to the first friction shoe and secondfriction shoe, respectively, for urging the friction shoes toward theretracted positions and yieldably opposing radial extension of the firstfriction shoe and the second friction shoe, respectively.
 7. Theimprovement as set forth in claim 1, wherein each clutch comprises: aone-way slip clutch assembly, the slip clutch assembly including acoupling collar mounted on one of the spur gears, a roller bearing racemounted on the clutch collar, and clutch slips mounted for pivotalmovement on the roller bearing race; and, the clutch bell including aninsert collar that is insertable into each coupling collar and isengagable with the pivotal slips when the clutch collar is in theforward drive position and in the reverse drive position.
 8. Theimprovement as set forth in claim 1, including: servo-apparatus coupledto the clutch bell for moving the clutch bell along the main drive shaftto the forward drive position and to the reverse drive position.
 9. In ascale model vehicle including a chassis, a main drive shaft mounted forclockwise and counterclockwise rotation on the chassis, drive wheelscoupled to the main drive shaft for propelling the vehicle in forwardand reverse directions, an internal combustion engine having a rotarypower output shaft, and a transmission assembly coupled to the rotarypower output shaft for transmitting torque from the engine to the maindrive shaft, the transmission assembly comprising: inertial lock-outapparatus movably coupled to at least one of the first and secondclutches for radial extension into a blocking position which preventsmovement of the clutch bell from one drive position to the other driveposition when the engine is operating above a predetermined idle speed.